The Glock 29 is a great pistol for what it's designed for: A reasonably concealable pistol with 10mm power. I don't know if a mdl 20 barrel or one of the 6" aftermarket ones will work in the mdl29. They likely will, although it wouldn't be something that would be very practical, IMO. I would think accuracy would suffer with that much unsupported barrel protruding from the slide, and the short sight radius. The magazines from the mdl 20 will work. I'd bet someone makes sleeves for them to give a mdl 20 length grip. If I'm understanding you correctly, this is to be used as a trail gun, with occasional hunting use. As concealability is not normally a concern in those applications, my choice would be the mdl 20, which with decent sights, ( And maybe a 6" barrel) would fit your applications much better than the mdl 29.
Savage